Coke oven gas, like natural gas and refinery gases, normally contains relatively large amounts of hydrogen sulfide. When the gases are subsequently treated, either to form a synthesis gas or to be distributed and burned, the hydrogen sulfide has to be removed. When gases are desulfurized by the so-called oxidation method, hydrogen sulfide is oxidized to elemental sulfur in the liquid phase. In the neutralization method, in which the hydrogen sulfide occurs in the regeneration step, the hydrogen sulfide is converted to sulfuric acid in a sulfuric-acid plant or to elemental sulfur in a Claus plant. For economic reasons, the aforementioned Claus plants are usually two-stage, meaning that they have two catalytic stages.
Two-stage Claus plants reach a sulfur conversion rate of approximately 94%. They are, therefore, equipped with an afterburning chamber, in which the remaining hydrogen sulfide is burned to sulfur dioxide. In this manner, about 6% of the scrubbed hydrogen sulfide is released to the atmosphere in the form of sulfur dioxide which, of course, pollutes the atmosphere. For environmental reasons, therefore, this process is no longer acceptable.
In an effort to reduce the emission of sulfur dioxide into the atmosphere, it has been known to replace the afterburning unit in the aforementioned Claus plant with a hydrogenation stage which hydrogenates the sulfur compounds in the waste gas from the Claus plant to hydrogen sulfide with the use of coke oven gas. This rehydrogenated hydrogen sulfide is then scrubbed in a downstream hydrogen-sulfide scrubbing unit before the remaining gases are afterburned and introduced into the atmosphere as sulfur dioxide.